Tiering on Land – Trees and Forests (Late Palaeozoic)

Abstract

Trees with woody trunks first appeared in Middle Devonian times (390 MYA). However, a number of other types of Palaeozoic
plants also achieved large sizes, most notably the arborescent lycopsids. The tropical wetlands of late Carboniferous and
early Permian age (320–280 MYA), often referred to as the coal swamps or coal forests, were dominated by the arborescent lycopsids.
Because of the unusual growth and reproductive strategies of these plants, and the water‐logged and low pH substrates, the
ecological structure of these forests was quite different from most modern‐day forests. Rather than there being a vertical
stratification of the vegetation controlled by light availability, there was a lateral ecological partitioning of the habitats
controlled mainly by substrate conditions.

Key Concepts:

The late Palaeozoic coal forests were relatively open habitats, and therefore, light levels were not significant in controlling
their ecological partitioning.

Substrate conditions were most important for controlling plant distribution in the coal forests, resulting in a lateral ecological
partitioning, rather than a vertical tiering as is typical in most modern‐day forests.

The rapid determinate growth of the arborescent lycopsids that dominated the coal forests meant that huge amounts of peat
were generated, which are now preserved as coal.

The coal forests were a major carbon sink during the late Palaeozoic times and had a significant impact on levels of atmospheric
carbon dioxide and global climates.

Keywords: forests; Palaeozoic; Palaeobotany; trees

Figure 1.

Arborescent lycopsids growing in the wettest parts of the late Carboniferous coal forests. In the centre is a fully mature
plant with a large crown of branching shoots bearing cones. On either side are younger plants at various stages of development,
each consisting of an upright stem covered with leaves for much of its length. Illustration by Annette Townsend (Cardiff) and now stored in the Department of Biodiversity and Systematic Biology, National
Museum Wales, Cardiff. Copyright of National Museum Wales.

Figure 2.

Model of part of the trunk of an arborescent lycopsid (Lepidodendron). Far left segment shows leaves still attached. Centre left shows the trunk after leaves have been removed, leaving photosynthetic
leaf bases. Centre and far right show the trunk after different levels of bark have been shed. Modelled in wax by Annette
Townsend (Cardiff) and now stored in the Department of Biodiversity and Systematic Biology, National Museum Wales, Cardiff.
Copyright of National Museum Wales.

Figure 3.

Fossil of part of the bark from an arborescent lycopsid trunk (Lepidodendron), showing the leaf bases left after the leaves were shed. The specimen originated from the Pennant Formation, South Wales
(late Moscovian in age), and is now in the collections of the National Museum Wales, Cardiff. Copyright of National Museum Wales.

Figure 4.

Model of terminal leafy branches of a mature arborescent lycopsid. Modelled in wax by Annette Townsend (Cardiff) and now stored
in the Department of Biodiversity and Systematic Biology, National Museum Wales, Cardiff. Copyright of National Museum Wales.

Figure 5.

Model of the female cone of an arborescent lycopsid (Lepidocarpon). Also shown are some isolated sporophylls from such a cone, each of which bore a single functional spore. The sporophylls
would have detached themselves from the cone on maturity and aided the wind dispersal of the spore. Modelled in wax by Annette
Townsend (Cardiff) and now stored in the Department of Biodiversity and Systematic Biology, National Museum Wales, Cardiff.
Copyright of National Museum Wales.

Figure 6.

View of the vegetation growing on a raised levee in the late Carboniferous coal forests. In the background can be seen the
large stems of arborescent lycopsids growing in the wetter parts of forest. In the foreground are marattialean tree ferns
with an upright trunk and a crown of fronds. The undergrowth consisted of a combination of shrubby and herbaceous ferns, pteridosperms
and cordaites. Artwork by Annette Townsend (Cardiff) and now stored in the Department of Biodiversity and Systematic Biology,
National Museum Wales, Cardiff. Copyright of National Museum Wales.

Figure 7.

Reconstruction of a typical late Carboniferous horsetail (Calamites). These were characteristic of the banks of lakes and rivers of the tropical coal forests and were capable of regenerative
growth if the plants were flooded by mud. Drawing by Annette Townsend (Cardiff) and now stored in the Department of Biodiversity
and Systematic Biology, National Museum Wales, Cardiff. Copyright of National Museum Wales.

Figure 8.

Reconstruction of a typical late Carboniferous cordaite tree. Drawing by Annette Townsend (Cardiff) and now stored in the
Department of Biodiversity and Systematic Biology, National Museum Wales, Cardiff. Copyright of National Museum Wales.

Figure 9.

View of part of the coal forests, showing the backswamp vegetation dominated by arborescent lycopsids on the left and centre.
To the right is a river bounded by levees, growing on which were ferns, pteridosperms, cordaites and some sphenophytes. Artwork
by Annette Townsend (Cardiff) and now stored in the Department of Biodiversity and Systematic Biology, National Museum Wales,
Cardiff. Copyright of National Museum Wales.

Figure 10.

Palaeogeograhical map showing distribution of coal forests (green) during middle Moscovian times. Land areas shown in light
brown, upland areas in dark brown. Redrawn from Cleal and Thomas using a base map by C Scotese, with modifications after Laveine et al.. Copyright of National Museum Wales.

Figure 11.

Correlation of the geographical size of the coal forests (centre) with evidence of global temperatures, determined from the
size of the polar ice sheet (left) and vegetation patterns observed in the northern temperate and high latitudes. Modified from Cleal and Thomas .

Zhou Y‐X
(1994)
Earliest pollen‐dominated microfloras from the early late Carboniferous of the Tian Shan Mountains, NW China: their significance for the origins of conifers and palaeophytogeography.
Review of Palaeobotany and Palynology
81:
193–211.